Field of the Invention
This invention relates to a control system for drive transmissions and, more particularly, to a fluid pressure operated control system for preventing engagement of the transmission in an improper starting position.
Motorized vehicles and machinery including those used in agricultural and industrial type equipment often have one or more clutches and brakes which are actuated by fluid pressure in order to transfer torque between the power source and the driven device. A control system is usually provided with an electric neutral start switch for preventing starting of the vehicle or machinery until a neutral or, in the case of vehicles, a park position is selected for the drive train. However, operators may electrically bypass such a device when it is inconvenient to use the starter switch thereby allowing the vehicle or machinery to be started in a drive position. This can occur if an operator jump-starts a vehicle that has been left in gear and, upon starting, the vehicle moves in a forward or reverse drive. Numerous types of control systems incorporating an engagement override valve (EOV) for interrupting fluid flow to fluid actuated clutches when a vehicle has been started with a transmission in a drive condition are disclosed in the art.
EOV's for preventing drive from an engaged transmission when an engine is started have been incorporated into transmissions in various ways. U.S. Pat. No. 3,863,523 issued Feb. 19, 1975 to J. Starling et al teaches a complete transmission control system for controlling speed ratio devices and directional devices. The control system incorporates an EOV to ensure that no fluid can be supplied from a fluid source to actuate a speed ratio device unless the directional controls are in a neutral position. When the vehicle is started in a neutral condition, fluid pressure builds up and causes the EOV to shift to a position allowing full flow of fluid to the speed ratio device. The Starling system is complicated due to the presence of many valves and modulating devices for controlling the total operation of the transmission. Another complicated system is disclosed in U.S. Pat. No. 3,468,194 to Horsch. Horsch uses a speed selector valve, a directional selector valve, a pressure modulating valve and a differential valve to control the total function of a transmission. The differential valve in Horsch also includes an engagement override portion which, upon attempting a startup in a drive condition, acts to block flow to the directional selector valve and ultimately the directional clutches. A more simple control device which incorporates an EOV, and is particularly suited for preventing engagement of a clutch actuated transmission is U.S. Pat. No. 4,396,099 issued Aug. 2, 1983 to Shirley. This patent teaches an EOV which is shifted from an initial override position to an engagement position by temporarily redirecting fluid flow from a control valve directly linked to a clutch pedal. The Shirley patent is much simpler than the other references since the operation of the EOV and the clutch control valve are not tied to other transmission functions.
Although the above-described control systems will provide the necessary safety function under most conditions, these systems have often been avoided due to the complexity and cost involved. While the Shirley system is much simpler and therefore does not involve the cost disincentives, it along with the other systems have the drawback that the EOV may shift to the override position during normal operation when high fluid demands cause low pressure within the fluid supply system.
Typically, an EOV is constructed such that it is moved to the operational position by build up of pressure on one side of a spool valve which is spring loaded on the opposite side to move the valve spool to an override position when fluid pressure is released. In many systems, the chamber for holding engagement pressure is in direct communication with the pressure source for the hydraulic circuit. Consequently, during periods of low pressure, the EOV can act as an accumulator which moves into the override position as fluid is supplied to lower pressure zones. This loss of pressure is particularly likely to occur in systems that use multiple clutch or brake elements.
This untimely disengaging action of the EOV, which will require additional clutching or movement of the shift lever back to neutral to regain drive train engagement, is annoying and disruptive to vehicle operations. Unexpected disengagement of the transmission causes the operator to lose vehicle power and control of forward or backward movement. Therefore, it is desirable to have a system that will maintain the EOV in an operational position during a temporary loss of sustaining pressure.
U.S. Pat. No. 3,709,065 issued Jan. 9, 1973 to Starling shows another transmission control system having an EOV which incorporates an orifice into the side of the EOV for retarding the disengagement function when the control system loses pressure. The orifice acts to prolong the period for disengagement of the EOV during periods of low system pressure by restricting fluid flow. However, the metering action of the orifice is not completely satisfactory. On one hand, a relatively large orifice may not provide sufficient time before the valve closes in which pressure may be reestablished in the system. Conversely, if the orifice is made very small it may take an inordinate amount of time for the EOV to reset to the override position.
An EOV that avoids the problem of untimely disengagement would be particularly useful in multispeed continuous drive transmissions. When operating such transmissions, certain speed selections may require the simultaneous filling of three or more clutch or brake elements. These large fluid demands can cause the low pressure condition which would move the EOV to an override position requiring shifting of the transmission back to a neutral position in order to reestablish drive. In addition, multispeed continuous drive transmissions do not require the use of the clutch pedal in order to engage the transmission. Therefore, a clutch activated EOV is unsuitable for these transmissions.